This invention relates to a liquid filter especially adaptable for suppressing infrared radiation. In a more particular manner, this invention relates to an ultraviolet light liquid chemical filter for use in suppressing and removing the excess heat, or infrared radiation, from a beam of light radiating from a simulated solar light source while simultaneously allowing the passage of ultraviolet radiation through the filter.
The present utilization of earth orbiting satellites, as well as other space vehicles and systems contemplated for use in the future, has generated considerable interest in establishing test and evaluation procedures for the materials used in their manufacture. These space vehicles often require the use of flexible, thin film materials on their external surfaces. Such materials are often employed as coverings in thermal control systems or as the principal operating component for devices which inflate, unfurl or otherwise expand during operation.
An important factor which must be considered by those using these thin film materials as structural components is the potential damage which can occur from natural space radiation. The physical and chemical properties and characteristics of the film can be seriously altered thereby affecting the operational efficiency of the space vehicle. This situation is compounded by the fact that the mean mission lifetime of space systems is increasing from 1 to 3 years for earlier spacecraft to 5 to 10 years for present and future spacecraft. Some requirements may even go as high as 30 years.
The critical space radiation environment consists primarily of solar ultraviolet radiation and charged particles (electrons and protons) which can cause both surface and bulk damage to the materials. The effects of space radiation on the properties of metals and inorganic materials are usually small. However, organic materials, like polymer films and adhesives, are subject to a wide range of radiation degradation effects depending on the molecular structure of the material and the quantity of energy absorbed from the radiation sources. The resulting formation of free radicals and ions in the material lead to cross-linking, chain scission, chain polymerization, block copolymerization, unsaturation, and chain transfer.
Typical manifestations of this energy interaction and deposition within the organic materials include outgassing (and contamination of surrounding surfaces), shrinkage, cracking, crazing, pitting, embrittlement, and discoloration. These cause degradation of useful mechanical properties such as tensile strength, elongation, and modulus of elasticity and degradation of important optical and thermophysical properties such as transmittance, reflectance, and solar absorptance.
Most of the accumulated radiation dosage near synchronous altitude is considered to be low energy and thus is absorbed in the surface of the material (&lt;0.1 mil penetration depth). Total surface dosage can be quite high--between 10.sup.9 to 10.sup.12 rads. For bulk penetration (0.1- to 10-mil depth), the absorbed dosage is between 10.sup.8 to 10.sup.9 rads. By contrast, threshold levels for incipient radiation damage in polymer films is on the order of 10.sup.5 to 10.sup.8 rads. In most applications, total thickness of the polymer films is in the one to 10 mil range; thus, a substantial fraction of the material is affected by the absorbed energy due to the synchronous radiation environment.
Therefore, it is extremely important that the critical properties and characteristics of these thin film materials, as altered or affected by long term exposure to the radiating environment of space, be fully understood. Otherwise, the proper evaluation of thin film materials for potential use within a space environment would be extremely difficult to say the least. The potentiallity for change in their physical and chemical characteristics must be known and evaluated for design purposes.
One particular problem associated with the use of thin filming materials concerns itself with the ability of these materials to withstand the degradative effects of ultraviolet light radiation which occurs on exposure to sunlight. In order to ascertain the degree of resistance to ultraviolet light, tests have been devised that simulate exposure to solar radiation. However, attempts at speeding up the testing procedure have proven to be unfruitful since the incident radiation resulting from accelerated testing procedures is at a much higher solar flux level than exists under actual operating conditions. This condition, unfortunately, causes the materials to heat up excessively and thereby disguises their true response to the effects of radiation and provides the test analyst with misleading information. The overheating is caused by the visible and infrared portions of the solar spectrum. Damage inflicted on the thin film material, however, results largely from ultraviolet radiation. Accordingly, an optimum testing procedure would be one which reflects or rejects the heat producing portion of the spectrum while, at the same time, allowing a major proportion of the ultraviolet portion to impinge upon a test sample in order to determine the sample's resistance to ultraviolet light degradation. In other words, interposing a proper filter between a source of solar radiation and the test sample could eliminate the undesirable heating effects.
In attempting to provide a solution to the problems associated with ultraviolet radiation resistance testing, it was found that a cobalt sulfate liquid filter having a path length of about 10 cm provided an effective means for eliminating much of the undesired heat produced from a solar or simulated solar radiation source during materials testing programs.